Visual studio has problems with duplicate filenames. In this case posix.c. They are both needed (the win32 version and the normal version) and generate the same output file when they're both in the same project. I can work around it, but it is not nice in Visual Studio. You have to make win32/posix.c compile to another directory. It can also be solved by putting an ugly #include "../posix.c" in the win32/posix.c version and leave the other posix.c out. The best way imho however is to rename the win32/posix.c file.